The present invention relates to a color cathode ray tube which is equipped with an electromagnetic field leakage preventing coating, and, more particularly, to a color cathode ray tube having a field leakage preventing coating in the form of a double coating film composed of a conductive high-refractive first layer and a low-refractive second layer adhering on the surface of a faceplate of a panel section of the cathode ray tube envelope.
There is a conventional cathode ray tube in which a double coating film, composed a high-refractive first layer and a low-refractive second layer, is adhered on the outer surface of a faceplate of a panel section of the tube envelope to prevent glare on the faceplate, to prevent the accumulation of an electric charge which may produce an electrical shock when touching the faceplate and to provide an improved contrast in a displayed image produced by the cathode ray tube.
Here, the anti-reflection function of the cathode ray tube having the conventional double coating film is achieved by interference of light caused by the double coating film.
The electrical charge preventing function for preventing an electrical shock when touching the cathode ray tube having the conventional double coating film is achieved by reducing the surface sheet resistance (hereinafter abbreviated as a surface resistance) of the conductive high-refractive first layer to 10.sup.4 to 10.sup.8 .OMEGA./.quadrature. by using a conductive high-refractive first layer, into which conductive particles, including tin oxide (SnO.sub.2) and antimony oxide (Sb.sub.2 O.sub.3), are combined, or into which conductive particles, including tin oxide (SnO.sub.2) and indium oxide (In.sub.2 O.sub.3), are combined, for example.
Further, the function for providing a high contrast in the image displayed by the cathode ray tube having the conventional double coating film is achieved by mixing a certain amount of pigment of a specific color into the high-refractive first layer of the double coating film.
As the surface treatment film formed on the outer face of the faceplate of the panel section of such a color cathode ray tube, there have been known 1) one which is obtained by forming a conductive film having a resistance of about 1.times.10.sup.3 .OMEGA./.quadrature. by means of sputtering, evaporation or the like to prevent electrical charge accumulation and to suppress leakage of electromagnetic waves, and by forming thereon a multi-layered film in which a low-refractive film and a high-refractive film are laminated, 2) one which is obtained by forming a NESA coating film on the outer face of the panel by means of CVD or the like as a conductive film and by laminating thereon a high-refractive film, or 3) one which is obtained by forming a conductive film by applying a solution in which particles of silver (Ag), whose specific resistance is low, are dispersed by application means, such as spin coating, and by forming thereon a low-refractive layer made of silica (SiO.sub.2) by means of spin coating or the like.
It is noted that the technological features for achieving the above-mentioned functions have been disclosed in Japanese Patent Laid-Open Nos. Hei. 3-93136, Hei. 5-113505, Hei. 5-343008 and Hei. 7-312170.
In addition, there also is a known cathode ray tube which prevents an electric field generated within the cathode ray tube from leaking from the outer face of the faceplate of the panel section, i.e. a cathode ray tube having a field leakage preventing coating film, provided by adhering and forming a double coating film composed of a high-refractive first layer and a low-refractive second layer on the outer face of the faceplate of the panel section of the tube envelope and by using a conductive high-refractive first layer, in which metal particles are mixed, as the high-refractive first layer to reduce the surface resistance of the conductive high-refractive first layer to less than 1.times.10.sup.3 .OMEGA./.quadrature., and an example thereof is described in a magazine "Industrial Material" vol. 44, No. 9 (August, 1996) pp. 68-71.
By the way, the above-mentioned cathode ray tube having the double coating film has had a problem in that, although a conductive high-refractive first layer into which conductive particles, such as tin oxide (SnO.sub.2) and antimony oxide (Sb.sub.2 O.sub.3), are combined, or conductive particles, such as tin oxide (SnO.sub.2) and indium oxide (In.sub.2 O.sub.3), are combined, for example, is used, it has not been possible to prevent the electric field generated within the cathode ray tube from leaking to the outside through the outer face of the faceplate of the panel section on which the conductive high-refractive first layer is provided because the surface sheet resistance of the conductive high-refractive first layer is 1.times.10.sup.4 to 10.sup.8 .OMEGA./.quadrature..
The prior art cathode ray tube having the field leakage preventing coating film using particles of silver (Ag) also has had a problem in that, the film does not provide enough mechanical strength.
The prior art cathode ray tube having a field leakage preventing coating film has had another problem in that, although it allows a high contrast to be obtained by suppressing the black color of the body of the cathode ray tube, which is caused by the double coating film composed of a conductive high-refractive first layer and a low-refractive second layer, along with an increase of reflection of external light on the fluorescent screen within the faceplate from standing out due to the light absorptivity of the metal particles mixed into the conductive high-refractive first layer, the body color of the cathode ray tube is colored to a color other than black because the spectral transmittance of the mixed metal particle layer differs depending on the wavelength of the light. For instance, when the spectral transmittance of the layer of the mixed metal particles is low, such as around 420 nm in wavelength, and shows a peak of absorption, the double coating film composed of a high-refractive first layer and a low-refractive second layer turns out to have an amber color, a hue inadequate for the display.
The prior art color cathode ray tube having the structure 1) described above has had a problem in that, although it effectively prevents glare and the accumulation of an electric charge and suppresses the leakage of electromagnetic waves, its production cost is remarkably high. The color cathode ray tube having the structure 2) described above has had a problem in that it requires a number of steps for forming the NESA coat and the multi-layered film and that a desired performance cannot be fully obtained. The color cathode ray tube having the structure 3) described above has had a problem in that it is difficult to maintain the initial performance for a long period of time, though its production cost is low.